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## 1. **Shallow copy**

Creates a new object, but **nested objects are copied by reference**.

---

### Way 1: Spread (`...`)

```javascript
const user = { name: 'Tim', age: 25 };
const clone = { ...user };

console.log(clone); // { name: 'Tim', age: 25 }
console.log(clone === user); // false
```

Fast, convenient, and modern.
But nested structures (objects, arrays) remain **references** to the original:

```javascript
const user = { name: 'Tim', address: { city: 'Kyiv' } };
const clone = { ...user };

clone.address.city = 'Lviv';
console.log(user.address.city); // "Lviv" (affects the original)
```

---

### Way 2: `Object.assign()`

```javascript
const user = { name: 'Tim', age: 25 };
const clone = Object.assign({}, user);

console.log(clone); // { name: 'Tim', age: 25 }
```

Same as spread, also a **shallow copy**.

---

### Way 3: Via a loop (manual way)

```javascript
const clone = {};
for (const key in user) {
  if (user.hasOwnProperty(key)) {
    clone[key] = user[key];
  }
}
```

Old-fashioned, but works even in old browsers.

---

## 2. **Deep copy**

Creates a fully independent copy -
even **nested objects are not linked** to the original.

---

### Way 1: `structuredClone()` (modern and safe)

```javascript
const user = {
  name: 'Tim',
  address: { city: 'Kyiv', zip: 12345 }
};

const clone = structuredClone(user);

clone.address.city = 'Lviv';

console.log(user.address.city); // "Kyiv"
```

Supported in all modern browsers and Node.js 17+.
It copies:

- nested objects, arrays, dates, Map, Set, and so on;
- it does not copy functions and prototypes (they are lost).

---

### Way 2: Via `JSON.parse(JSON.stringify())`

```javascript
const user = { name: 'Tim', address: { city: 'Kyiv' } };
const clone = JSON.parse(JSON.stringify(user));

clone.address.city = 'Lviv';
console.log(user.address.city); // "Kyiv"
```

Works almost everywhere, but:

- functions, `undefined`, `Symbol`, `Date`, `Map`, `Set`, and `BigInt` **are lost**;
- it can be slow on large structures.

---

### Way 3: Libraries (lodash, Ramda, etc.)

```javascript
import _ from 'lodash';

const clone = _.cloneDeep(user);
```

The most reliable way for complex structures -
it preserves types, nesting, and runs fast.

---

## SUMMARY

| Copy type | Way | Copies nested objects? | Preserves types (`Date`, `Map`, `Set`)? |
|---|---|---|---|
| Shallow | `{ ...obj }`, `Object.assign()` | No | Yes |
| Deep | `structuredClone()` | Yes | Yes |
| Deep (old way) | `JSON.parse(JSON.stringify())` | Yes | No |
| Deep (reliable) | `_.cloneDeep()` | Yes | Yes |

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